WritableStreamDefaultWriter: свойство closed

const writableStream = new WritableStream(
  {
    start(controller) {},
    write(chunk, controller) {
      // ...
    },
    close(controller) {
      // ...
    },
    abort(err) {
      // ...
    },
  },
  queuingStrategy,
);

// ...

const writer = writableStream.getWriter();

// ..

// check if the stream is closed
writer.closed.then(() => {
  console.log("writer closed");
});